1. Field of the Invention
The present invention relates to a method of starting an electric brushless rotating machine and particularly to a method of starting an electric brushless rotating machine appropriated for generating a large torque at the startup stage.
2. Description of the Related Art
A brushless motor is provided as an electric rotating machine where the energization of three-phase stator windings for driving a rotating member (referred to as a rotor hereinafter) is switched from one to another whenever the rotor rotates through 120 degrees of the electric angle. Such a conventional brushless motor has commonly a position detector element such as a Hall device for detecting the rotating position of the rotor. Recently, another type of brushless motor which includes no position detector element has been developed in response to the demand for down-sizing of the brushless motor.
For example, a brushless motor is disclosed in Japanese Patent Publication (Heisei)5-24760 where, in view of any two different phases of the three-phase stator windings being energized in a sequence, the voltage induced at the remaining not-energized phase is measured and used for calculating the rotating position of the rotor. As the brushless motor produces non of the induced voltage at the startup stage which is used for calculating the rotating position of the rotor, its rotor has slightly be driven by forced commutation. The forced commutation means that any two desired phases of the stator, e.g. U and V, are energized regardless of the position of the rotor (which is hence referred to as one-phase energization hereinafter). The position of the rotor is detected from the induced voltage and then a common procedure of the energization will follow in relation to the detected rotor position.
The positional relationship between the rotor and the stator when they stop their movement as the motor has been deenergized is determined by attracting and repulsing forces of the magnets. For example, when the motor is an outer rotor type brushless motor having three-phase stator windings, its positional relationship between the rotor and the stator is expressed by six different pausing modes, p1 to p6, shown in FIG. 13. FIG. 13 illustrates an arrangement of a primary part of the brushless motor in addition to the six pausing modes of the position relationship between the rotor and the stator of which the movement stops as the motor has been deenergized.
As shown in FIG. 13, the counter clockwise direction is the forward direction Rs of the rotor while the clockwise direction is the reverse direction Rr. The stator 100 and the rotor 200 of the brushless motor are disposed inward and outward respectively. The stator 100 has magnetic poles 300 of U, V, and W phase. The magnetic poles 300 incorporate windings. The rotor 200 has a row of permanent magnets m1, m2, m3, . . . of which the polarity alternates between N and S along the circumference.
A movement of the rotor from the initial pausing mode p1 to p6 when is driven by forced commutation between U phase and W phase without initial magnetization will be explained. When an electric current is supplied through U phase to W phase, the U phase is magnetized to positive (N) pole and the W phase is magnetized to negative (S) pole.
At the initial pausing mode p1, the magnet m2 at S is attracted by the U phase at N but repulsed by the W phase at S. This causes the rotor 200 to rotate at a maximum torque in the forward direction Rs. At the initial pausing mode p2, the U phase at N attracts the magnet m2 at S but repulses the magnet m3 at N hence allowing the rotor 200 to rotate at the maximum torque in the forward direction Rs. At the initial pausing mode p3, the attraction between the U phase at N and the magnet m2 at S is balanced with the attraction between the W phase at S and the magnet m1 at N. This permits no movement of the rotor 200.
At the initial pausing mode p4, the magnet m2 at N is attracted by the W phase at S while the magnet m1 at S is repulsed by the same. This causes the rotor 200 to rotate in the reverse direction Rr. At the initial pausing mode p5, the U phase at N attracts the magnet m3 at S but repulses the magnet m2 at N hence allowing the rotor 200 to rotate further in the reverse direction Rr. At the initial pausing mode p6, the repulsion between the U phase at N and the magnet m2 at N is balanced with the repulsion between the W phase at S and the magnet m1 at S. This permits no movement of the rotor 200.
As described, the startup torque may be generated non or too small at the initial pausing modes p3 and p6 thus disallowing the brushless motor to start up. In particular, when the brushless motor is linked to a heavy load and thus required to generate a large torque, this disadvantage will be significant. For example, the motor for starting an internal combustion engine, even if its output is great, may fail to generate a desired level of the startup torque because the friction in the engine is too high. At the initial pausing modes p4 and p5, the rotor rotates in the reverse direction and fails to generate a desired magnitude of the induced voltage needed for detecting the position of the rotor, hence inhibiting any normal energizing action. More particularly, by force commutation, the motor when remains free in the movement can be rotated in the forward direction two times out of six trials or at ⅓ of the probability.
It is hence an object of the present invention to provide a method of starting an electric brushless rotating machine which can generate a great level of the startup torque with no use of rotor position detecting elements. Another object of the present invention is to provide a method of starting an electric brushless rotating machine which can shift from the force commutation to a common operation simply and smoothly.
It is a further object of the present invention to provide a method of starting an electric brushless rotating machine which can continue to supply an upper limit level of current during the forced commutation thus to generate a climb over torque.
A first feature of the present invention is that a method of starting an electric brushless rotating machine for driving an internal combustion engine which has a magnetic rotor joined to an output shaft of the internal combustion engine and a set of stator windings of a first phase, a second phase, and a third phase arranged at equal phase intervals of an electric angle of 120 degrees so that the stator windings are energized in a sequence for forced commutation according to a rotating position detecting signal from the rotor, comprising the steps of, energizing between any two of the first, second, and third phase stator windings for initial magnetization at the startup to hold the magnetic rotor at a position, carrying out the forced commutation to energize the windings of the phases in a sequence while gradually increasing the level of the energization for forcefully rotating the magnet rotor, and generating the rotating position detecting signal from a voltage signal induced on the not-energized windings during the forced commutation and carrying out a normal action of the energization based on the rotating position detecting signal thus allowing the magnetic rotor to drive the output shaft of the internal combustion engine, and canceling the energization when the number of revolutions or the full turning motion in the internal combustion engine determined from the rotating position detecting signal reaches its predetermined level or times.
According to this feature, the internal combustion engine can be started up by a large level of the startup torque with no help of position detecting elements. The electric brushless rotating machine can hence be used as a brushless starter motor.
A second feature of the present invention is that the timing for switching from the forced commutation to the normal energization is taken when the number of revolutions or the full turning motion determined from the rotating position detecting signal reaches its predetermined level or times.
According to this feature, when the number of revolutions of the internal combustion engine has reached a specific number or the relationship between the commutation and the revolution has turned to substantially a stable state, the operation is automatically switched to the normal mode. This allows the switching of the mode to be executed simply and smoothly.
A third feature of the present invention is that the duty of PWM during the energization after the completion of the initial magnetization is gradually increased with a limiter arranged for limiting the energizing current to a specific level.
According to this feature, the startup to a desired number of revolutions can smoothly be conducted while the capacity of a driver for energization remains minimized.
A fourth feature of the present invention allows the energizing current to be gradually increased in the amount for the initial magnetization while being monitored not to exceed a predetermined level and when reaching the level, to be held at its level for continuous energization.
According to this feature, the upper limit level of the current can continuously be supplied during the forced commutation. As a result, the method of starting an electric brushless rotating machine can produce a greater startup torque. When the climb over torque is required for starting an internal combustion engine of a large startup torque type, it can simply be provided with no use of position sensors.
A fifth feature of the invention is that the rotating position detecting signal is determined from a voltage signal induced on the windings of the magnetic rotor not energized by the forced commutation and when the number of revolutions or the fully rotating motion reaches its predetermined level or times, the rotating position detecting signal is used for controlling the energization to the winding of each phase.
According to this feature, when the number of revolutions or the full turning motion reaches its predetermined level or times, i.e. the relationship between the commutation and the rotation becomes at a degree of stability, the operation can automatically be switched to a normal action by the switching based on the number of revolutions.
A sixth feature of the invention is that the energization is canceled when the energizing current exceeds the predetermined level.
According to this feature, the initial magnetization can be prevented from overload operations.
A seventh feature of the present invention allows the energization to each phase winding to be controlled through quantitatively increasing or decreasing the duty of PWM.
According to this feature, the energization to each phase winding can be controlled by a simple means.